3 Answers
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Goeschel’s study of suicide in Nazi Germany (2009) shows that the number of suicides varied greatly as the War progressed, as far as can be determined. Following the First World War the German suicide rate climbed steadily from just under 16 per 100,000 to 29 by the early 1930s

I happened to have access to the book Suicide in Nazi Germany, so just to corroborate the earlier answers by reproducing some figures from the book.

From p. 209 and p. 213, we see that suicide rates in Germany throughout the 1930s was indeed around 27 - 29:

Bonus: He doesn't give corresponding figures for the war years for Germany as a whole, but does for Berlin (p. 220). It looks like Berlin's suicide rates were always higher than Germany's, but spiked up in 1945.

By the way, 18000 is perhaps not such a large number. There were about that many each year around 1980 (when Germany's population was comparable to 1930s Germany).